Lightning is the most underrated weather hazard. It is the third greatest storm-related killer in the United States and causes nearly $1 billion in damages each year. Lightning makes every single thunderstorm a potential killer, whether the storm produces one single bolt or 10 thousand bolts.

In the United States, there are about 25 million lightning flashes every year. In 2012, 28 people lost their lives and hundreds of people were permanently injured by strikes. People struck by lightning suffer from a variety of long-term, debilitating symptoms, including memory loss, attention deficits, sleep disorders, chronic pain, numbness, dizziness, stiffness in joints, irritability, fatigue, weakness, muscle spasms, depression, and more.

Understanding the dangers of lightning is important so you can get to a safe place when thunderstorms threaten. If you hear thunder—even a distant rumble or a crackling aloft—you are already in danger of becoming a lightning victim.

How Thunderstorms Develop

Thunderstorms often begin to develop early in the day when the sun heats the air near the ground and pockets of warmer air start to rise in the atmosphere. When these pockets of air reach a certain level in the atmosphere, cumulus clouds start to form. Continued heating causes these clouds to grow vertically into the atmosphere. These “towering cumulus” clouds may be one of the first signs of a developing thunderstorm. The final stage of development occurs as the top of the cloud becomes anvil-shaped.

As a thunderstorm cloud grows, precipitation forms within the cloud. A well-developed thunderstorm cloud contains mostly small ice crystals in the upper levels, a mixture of small ice crystals and small hail in the middle levels, and a mixture of rain and melting hail in the lower levels of the cloud.

Air movements and collisions between the various types of precipitation in the middle of the cloud cause the precipitation particles to become charged. The lighter ice crystals become positively charged and are carried upward into the upper part of the storm by rising air. The heavier hail becomes negatively charged and is either suspended by the rising air or falls toward the lower part of the storm. These collisions and air movements cause the top of the thunderstorm cloud to become positively charged and the middle and lower part of the storm to become negatively charged.

The electric field within the storm is not the only one that develops. Below the negatively charged storm base, positive charge begins to pool within the surface of the earth. This positive charge will shadow the storm wherever it goes and is responsible for cloud-to-ground lightning. However, the electric field within the storm is much stronger than the one between the storm base and the earth’s surface, so most lightning (approximately 75-80 percent) occurs within the storm cloud itself.

How Lightning Forms

Lightning is a giant spark of electricity in the atmosphere or between the atmosphere and the ground. In the initial stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground; however, when the differences in charges become too great, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning.

Lightning can occur between opposite charges within the thunderstorm cloud (intra-cloud lightning) or between opposite charges in the cloud and on the ground (cloud-to-ground lightning).

What is Thunder?

Thunder is the sound made by a flash of lightning. As lightning passes through the air, it heats the air quickly. This causes the air to expand rapidly and creates the sound wave we hear as thunder. Normally, you can hear thunder about 10 miles from a lightning strike. Since lightning can strike outward 10 miles from a thunderstorm, if you hear thunder, you are likely within striking distance of the storm.

Lightning Safety

There is no safe place outside when thunderstorms are in the area. If you hear thunder, you are likely within striking distance of the storm. Too many people wait far too long to get to a safe place when thunderstorms approach. Unfortunately, these delayed actions lead to many of the lightning deaths and injuries in the U.S.

The best way to protect yourself from lightning is to avoid the threat. You simply don’t want to be caught outside in a storm. Have a lightning safety plan—a place to shelter—and cancel or postpone activities early if thunderstorms are expected. Monitor weather conditions and get to a safe place before the weather becomes threatening. Substantial buildings and hard-topped vehicles are safe options. Rain shelters, small sheds, and open vehicles are not safe.

While no place is 100 percent safe from lightning, some places are much safer than others.

If you hear thunder, get inside a safe place immediately; avoid contact with plumbing and anything plugged into an electrical outlet; stay off corded phones; and stay away from windows and doors. Remain there for 30 minutes after the last lightning or thunder.

Lightning is one of the most capricious and unpredictable characteristics of a thunderstorm. It is the first thunderstorm hazard to arrive and the last to leave. Remember, YOU are ultimately responsible for your personal safety and should take appropriate action when threatened by lightning.